1. Field of the Invention
This invention relates to the field of optical devices that manipulate light of tightly controlled wavelength, particularly for use in communication applications. More particularly, the invention relates to lasers that produce light of a specified wavelength and which can be tuned or switched to other specified wavelengths by thermal means.
2. Description of the Related Art
Over the past several years, there has been ever increasing interest in tunable lasers for use in optical communication systems, in general, and for use in dense wavelength division multiplexing (DWDM) applications, in particular. DWDM allows high bandwidth use of existing optical fibers, but requires components that have a broad tunable range and a high spectral selectivity. Such components should be able to access a large number of wavelengths within the S-band (1490-1525 nanometers), the C-band (1528-1563 nanometers), and the L-band (1570-1605 nanometers), each different wavelength separated from adjacent wavelengths by a frequency separation of 100 GHz, 50 GHz, or perhaps even 25 GHz, according to the system implementation.
The distributed Bragg reflector (DBR) laser was one of the first tunable lasers used in optical communication. The DBR laser consisted of a semiconductor amplifier medium, defining an active section, and an optical waveguide. The optical waveguide included a portion without a grating that defined a phase control section and a portion in which a single grating of typically constant pitch (xcex9) was formed which constituted a distributed Bragg reflector or, more simply, the Bragg section, that reflected light at the Bragg wavelength xcexB. The optical waveguide was defined by an organic layer which constituted a core with another organic confinement layer disposed both above and below the core. Wavelength tuning of such a DBR laser was performed by either injecting current or transferring heat into the phase control section, the Bragg section, or both. Injecting current made it possible to vary the refractive index of the waveguide and thus control the Bragg wavelength xcexB by the equation xcexB=2neffxcex9 where xcex9 is the pitch of the grating and neff is the effective refractive index of the waveguide. Alternatively, a pair of heating resistance strips were disposed on opposite outer surfaces of the laser component at the phase control section, the Bragg section, or both. By independently controlling the voltages to the resistance strips, the temperature and hence the index of refraction of the organic layers that form the optical waveguide was controlled via the thermo-optical effect. The DBR laser was continuously tuned over a small wavelength tuning range of approximately 10 nanometers. For a more detailed discussion of a wavelength tunable DBR laser by heating, please refer to U.S. Pat. No. 5,732,102 by Bouadma entitled xe2x80x9cLaser Component Having A Bragg Reflector of Organic Material, And Method of Making Itxe2x80x9d which is hereby incorporated by reference.
The DBR laser with selectively activated virtual diffraction gratings (Variation DBR Laser) was a variation of the DBR laser that employed current injection as the mechanism for wavelength tuning. The Variation DBR Laser replaced the single Bragg grating associated with a conventional DBR laser with a plurality of selectively activated virtual diffraction gratings. Specifically, the Variation DBR Laser included a plurality of periodic arrangements that constituted a plurality of virtual diffraction gratings. Each virtual diffraction grating had a different Bragg wavelength and hence a different wavelength tuning range. Injecting current into a first periodic arrangement created a first diffraction grating with a first Bragg wavelength which made it possible to vary the refractive index of the waveguide and wavelength tune the laser around the first Bragg wavelength. The switching of the injection current from the first periodic arrangement to a second periodic arrangement replaced the first diffraction grating with a second diffraction grating that had a second Bragg wavelength which made it possible to wavelength tune the laser over a range of wavelengths around the second Bragg wavelength. The Variation DBR Laser could be discontinuously tuned (in jumps) over a wavelength range several times the tuning range associated with the DBR laser. However, tuning by injection current had the disadvantage of increased optical cavity loss, increased optical noise, low output power, and the tradeoff between tuning and loss. For a more detailed discussion of a variation DBR by injection current, please refer to U.S. Pat. No. 5,581,572 by Delorme et al. entitled xe2x80x9cWavelength-Tunable, Distributed Bragg Reflector Laser Having Selectively Activated, Virtual Diffraction Grating.xe2x80x9d Further, for a discussion on tunable lasers in general, please refer to a paper by Rigole et al. entitled xe2x80x9cState-of-the-art: Widely Tunable Lasers,xe2x80x9d SPIE, Vol. 3001, Pages 382-393, 1997. Both the Delorme patent and the Rigole paper are hereby incorporated by reference.
The present invention relates to a distributed Bragg reflector laser whose wavelength tuning range is comparable to that of the Variation DBR Laser but does not suffer the shortcomings associated with using injection current as the mechanism for wavelength tuning.
In the laser according to the invention, the mechanism for wavelength tuning is the changing of temperature and hence the refractive index of thermo-optical material adjacent to periodic arrangements. Thermo-optical material has a large dn/dt, that is, a change in temperature of the thermo-optical material will substantially change the refractive index of the thermo-optical material. Further, the thermo-optical material has a large dn/dt over a large temperature range which allows for a large potential tuning range.
According to the invention, changing the temperature of the thermo-optical material adjacent to a chosen periodic arrangement changes the refractive index of the thermo-optical material such that the refractive index of the thermo-optical material begins to differ from the refractive index of the adjacent periodic arrangement. This difference in refractive index creates or activates a diffraction grating with a Bragg wavelength which is associated with the chosen periodic arrangement. In the current industry jargon, the diffraction grating is xe2x80x9cweaklyxe2x80x9d on and will reflect light at the Bragg wavelength.
Alternatively, when the thermo-optical material adjacent to the chosen periodic arrangement is at an off temperature, the refractive index of the periodic arrangement and the adjacent thermo-optical material are nearly the same, no diffraction grating is created or activated (within a tolerance factor for variations in refractive index), and no light is reflected.
As the temperature in the thermo-optical material adjacent to the chosen periodic arrangement continues to increase or decrease beyond the off temperature (depending on the composition of the thermo-optical material), the difference in the refractive index between the chosen periodic arrangement and the adjacent thermo-optical material also increases. This increased difference in the refractive index increases the reflectivity of the diffraction grating associated with the chosen periodic arrangement and also shifts the Bragg wavelength. In the current industry jargon, the diffraction grating is xe2x80x9cstrengthened.xe2x80x9d
Switching (the activation) from a first to a second diffraction grating requires changing the temperature of the thermo-optical material adjacent to the first periodic arrangement back to the off temperature and changing the temperature of the thermo-optical material adjacent to the second periodic arrangement to a temperature other than the off temperature. This de-activates and hence eliminates the first diffraction grating and creates and hence activates the second diffraction grating with a second Bragg wavelength.
Typically, a single diffraction grating is activated at a given time. Each Bragg wavelength associated with each diffraction grating differs from all others. The range of wavelength tuning associated with a given diffraction grating is such that there is little or no overlap with any other wavelength tuning range of any other diffraction grating. Thus, by properly modulating the activation of each periodic arrangement, the laser has a very large tuning range associated with the net wavelength coverage of all the grating tuning ranges.
In the laser according to this invention, the periodic arrangements are adjacent to a core of a waveguide or, alternatively, the periodic arrangements are part of a layer which is adjacent to the core of the waveguide. Each periodic arrangement has a period which corresponds to the pitch of the associated diffraction grating when activated. In some embodiments, all the pitches are the same and, in other embodiments, each pitch is different. More specifically, a laser according to this invention includes a gain means with an active emission section which is optically coupled to a core of a waveguide and a substrate which supports both the gain means and the waveguide. More than one periodic arrangement is formed adjacent to the core or, alternatively, a layer which includes more than one periodic arrangement is formed adjacent to the core. Thermo-optical material is adjacent to each periodic arrangement and temperature changing means are disposed in the thermo-optical materials that are adjacent to each periodic arrangement. When the thermo-optical material adjacent to a periodic arrangement is at an off temperature, the refractive index of the thermo-optical material adjacent to the periodic arrangement is the same as the refractive index of the periodic arrangement and no diffractive grating is created or activated.
Between the gain means and the periodic arrangements along a periodic arrangement free-portion of the core, there may be a phase control section which can slightly shift the cavity modes associated with the laser. Also, beneath the core and associated with each periodic arrangement, the substrate may include index loading regions so that there is a different effective index of the optical mode for each periodic arrangement when the period of all the periodic arrangements is the same.
The thermo-optical material of the tunable laser is preferably selected so as to have a high coefficient of variation in refractive index as a function of temperature, the magnitude of which should be preferably greater than 1xc3x9710xe2x88x924/xc2x0 C. Examples of thermo-optical material used in the laser and that exhibit these characteristics include polymers derived from methacrylate, siloxane, carbonate, styrene, cyclic olefin, or norbornene.
It should be observed that, except for the gain means, the laser is fabricated using Si processing technology and only the gain means is of GaAs, InP, InGaAsP, or other direct bandgap materials or gain mediums which requires complex and sensitive processing technology, such as epitaxial growth and cleaving. The gain means is independently fabricated with a minimum of structure. Accordingly, the laser is easy to manufacture, cost effective, and results in high yield.